For decades, it has been proposed to replace traditional webs (woven fabrics, knitted fabrics) by what are known as xe2x80x9cnonwovenxe2x80x9d structures which, in general terms, may be classified in three large categories by virtue of the actual process for manufacturing them, to be precise nonwovens known as xe2x80x9cby the dry wayxe2x80x9d and xe2x80x9cby the melted wayxe2x80x9d and nonwovens known as xe2x80x9cby the wet wayxe2x80x9d.
In the rest of the description, the present invention will be described as applied to obtaining nonwovens by the technique known as xe2x80x9cby the dry wayxe2x80x9d, but it is clear that it is not limiting and that, if appropriate, it could be applied to the other two categories of nonwovens.
One of the main problems which arises during the production of nonwovens is that of achieving the cohesion of the structure in order to give the products mechanical characteristics according to the application in question, while maintaining or imparting particular physical characteristics, such as bulk, handle, appearance, etc.
Among the techniques proposed hitherto, such cohesion is obtained by the intermingling of the fibers in the thickness of the web by means of the action of fluid jets and, more particularly, of jets of water under pressure.
Such a technique of treatment by means of water jets has been proposed for decades, as may be gathered from the U.S. Pat. Nos. 3,214,819, 3,508,308 and 3,485,706.
In general terms, this technique involves carrying out the interlacing of the elementary fibers with one another by means of the action of jets of water under pressure which act on the fibrous structure in the manner of needles and make it possible to reorient part of the fibers forming the web in the direction of thickness.
Such a technology has been widely developed at the present time and is used not only for producing what are known as xe2x80x9cspunlacexe2x80x9d structures for textile use, such as, in particular for applications in the medical and hospital fields, for wiping, filtration and wrappings for teabags, but also for making tiny perforations in continuous supports such as paper, cardboard, films, even sheets of plastic or the like and the articles obtained may be regular and homogeneous and take the form of a plane sheet which, if appropriate, may have perforations, as may be gathered from the patent U.S. Pat. No. 3,508,308, and even, if appropriate, comprise designs resulting from the reorientation of the fibers, this being essentially for an esthetic purpose, as may be gathered from U.S. Pat. No. 3,485,706.
As regards the applications of such products of the xe2x80x9cspunlacexe2x80x9d type, it has been known for a very long time that the final properties of the product obtained can be adapted by producing mixtures of material, for example by combining with one another a plurality of webs consisting of fibers of different types, for example of natural, artificial or synthetic fibers, even from webs in which the fibers are previously mixed, reinforcements (woven or nonwoven nettings, webs of the xe2x80x9cspunbondxe2x80x9d type, etc.) being capable, if appropriate, of being incorporated into the nonwoven structure.
Of the numerous applications of nonwovens, mention may be made of wiping products known as xe2x80x9cwet wipesxe2x80x9d, and of products in the hygiene field.
In addition to good mechanical characteristics both in length and in width, also both in the dry state and in the wet state, such products must also have the essential characteristic of having a good capacity for the absorption and retention of the liquid or treatment product which they support or which they must absorb. Moreover, they must have good dimensional stability and be flexible, soft, bulky and pleasant to handle.
The invention is particularly suitable for producing such articles, and, in the rest of the description, it will be described more particularly for obtaining such products.
Hitherto, to produce wiping cloths or wet wipes, it has been conceivable to use nonwoven webs consisting entirely of absorbent fibers, such as cellulose fibers, particularly viscose fibers. In order to give the product good mechanical characteristics, a mixture of synthetic fibers (polyester, polypropylene) and of artificial fibers (viscose) is preferably used.
To produce such articles consisting of a mixture of fibers, either an intimate mixture of said fibers can be produced, which is converted into a homogeneous unitary web, or the combination of two webs previously produced is carried out, to be precise, one web based on discontinuous fibers or on continuous synthetic filaments (polyester, polypropylene) and the second based on artificial fibers (viscose), and said webs are combined by hydraulic entanglement.
It has also been proposed, in order to produce such articles, to replace the artificial fibers which give the web the capacity for the absorption and retention of the liquid with natural cellulose fibers, such as wood fibers, incorporated in the same proportions as the prior products based on artificial or synthetic fibers, said natural cellulose fibers being entangled with the chemical fibers likewise by treatment by means of water jets.
Such a product has good mechanical resistance characteristics, essentially afforded by the synthetic fibers, and good liquid absorption and retention characteristics imparted by the cellulose fibers.
French patents FR-A-2 730 246 and 2 734 285, these patents corresponding respectively to the patents U.S. Pat. No. 5,718,022 and U.S. Pat. No. 5,768,756, describe solutions which make it possible to successfully treat hydrophobic fibers or mixtures of these fibers with other hydrophilic fibers or even webs consisting entirely of natural fibers by means of water jets.
In general terms, according to the teachings of these documents, the treatment involves treating a basic web composed of elementary fibers of the same type or of different types, compressing and moistening this basic web and then intermingling the fibers by means of at least one rack of contiguous jets of water under high pressure acting on the basic web.
For this purpose, the basic web is advanced positively on an endless porous support in motion, and it is brought onto the surface of a perforated rotary cylindrical drum, to the interior of which a partial vacuum is applied.
The basic web is compressed mechanically between the porous support and the rotary drum which both advance substantially at the same speed.
Immediately downstream of the compression zone, a water curtain is directed onto the web and passes successively through the porous support, the compressed basic web and the supporting perforated drum which sucks up the excess water.
The elementary fibers are intermingled continuously, still on the rotary cylindrical drum, by the compressed and wetted web being subjected to the action of at least one rack of jets of water under high pressure. In general, bonding is carried out by means of a plurality of successive racks of water jets which act either on the same face or alternately against the two faces of the web, the pressure within the racks and the velocity of the jets discharged varying from one rack to the next and usually progressively.
Moreover, it is appropriate to note, as may be gathered from FR 2 734 285, that the perforated roller preferably comprises randomly distributed micro-perforations.
If appropriate, after this bonding treatment, the nonwoven structure may be subjected to a second treatment applied to the reverse face.
A method has been found, this being the subject of the present invention, which not only makes it possible to produce absorbent nonwovens having excellent physical properties (tensile strength, tearing strength, abrasion resistance) and a good capacity for the absorption and retention of liquid similar to articles of the same composition obtained according to the teachings of FR-2 734 285 but which, moreover, has, in relation to such articles, a more pleasant handle and increased bulk.
In general terms, therefore, the invention relates to a method making it possible to produce a novel type of nonwoven consisting at least partially of hydrophilic elementary fibers, which involves continuously:
producing a fibrous web by carding or another conventional technique;
compressing and moistening said web;
subjecting the moistened and compressed web to a bonding treatment by means of water jets acting at least against one of its faces, the web being supported by a rotary drum comprising randomly distributed micro-perforations, a partial vacuum being applied to the interior of said drum;
transferring said bonded web to an assembly making it possible to drain and dry it before it is received, in particular in the form of a wound package.
The method according to the invention is characterized in that, after bonding treatment by means of water jets and before drying and reception, the prebonded and moistened web is transferred to an additional assembly for treatment by the action of one or more series of water jets, said assembly comprising a movable suction surface taking the form of a drum covered with an open-work structure consisting of a woven metal-wire cloth and shaped to have a succession of raised and recessed zones.
For implementing the method according to the invention, the first treatment by means of water jets is carried out according to the teachings of FR-A-2 730 246 and FR-A-2 734 285, the content of which is incorporated in the present description as required.
The additional treatment by means of jets which is carried out before the drying of the web is obtained, as mentioned above, by causing it to pass onto the surface of a perforated rotary cylindrical suction drum. In a known way, such a drum consists of a honeycomb structure which is covered with a perforated plate and which rotates about a second hollow fixed coaxial cylindrical drum connected to a partial vacuum source in order to form a suction box below the zones where the water jets act. According to the invention, said drum is covered with a likewise perforated or intrinsically porous structure having raised and recessed zones, this structure preferably consisting of a woven fabric taking the form of a removable sleeve.
By virtue of such a design, it is therefore easy, simply by changing this sleeve, to modify the appearance and characteristics of the product obtained.
As covering structure, preferably used for the suction drum will be a woven cloth which is produced, for example, from bronze wires, and which has the characteristic of being capable of being embossed.
In such a structure, the ratio of emptiness in relation to the surface is between 10% and 50%, and the overall height between the bottom of the recessed parts and the upper part of the raised zones is generally between 0.5 mm and 2 mm.
The configuration of the raised and recessed portions which such a cloth will have may be of any type, for example, may take the form of herringbones, of designs with exact contours (circular, square) or a crimped, crinkled, etc. appearance.
In a variant according to the invention, before treatment on the embossed surface, the complex may, if appropriate, receive a second bonding treatment by means of jets, which is carried out in a similar way to the preceding treatment, but on the opposite face.
After treatment, the nonwoven, still in the wet state, is transferred to a squeezing-out zone, followed by a drying zone consisting of a through-air drum. Finally, it is received in the form of a wound package.
Such a method can be implemented for fibrous webs of any composition produced from natural, artificial and/or synthetic fibers taken alone or in a mixture.
For the production of absorbent nonwovens, it is conceivable to produce a web consisting entirely of viscose fibers.
Preferably, however, a mixture of absorbent artificial fibers (viscose) and of synthetic fibers, such as polyester or polypropylene, will be used.
In such mixtures, the linear density of the elementary fibers and their length are preferably similar and, for example, are between 1 dtex and 3.3 dtex, the length being between 20 mm and 60 mm.
It is also conceivable to adapt the method so as to incorporate natural fibers, such as wood fibers, into the web, for example by pneumatic lapping followed by hydraulic bonding by means of racks of jets acting against the face on which the wood fibers were distributed.
This operation of pneumatic lapping and bonding is carried out before shaping treatment on the embossed assembly, the jets of this assembly preferably acting against the surface covered with wood fibers.